Support to purchase a Thermo Electron Delta V Advantage isotope ratio mass spectrometer and accessories is requested. The instrument is to update the mass spectrometers in the Gas-Isotope-Ratio Mass Spectrometry Laboratory (GIRMSL) at the USDA/ARS Children's Nutrition Research Center (CNRC). The current mass spectrometers in the GIRMSL were purchased between 1986 and 1990. The manufacturers are either no longer in business or no longer support the antiquated hardware and operating software. The Delta V instrument is the most advanced isotope ratio mass spectrometer system for the measurements of stable hydrogen and oxygen isotopes. These isotopes are the backbone of the doubly labeled water (DLW) method which is considered by the research community to be the reference method for the estimation of energy expenditure or caloric requirement under free-living conditions. With the obesity epidemic, many studies are using the DLW method because the method is non-invasive, portable, and safe. This makes the DLW method ideal for studies involving infants and children as well as pregnant and lactating women. With 18O-water becoming more readily available, we anticipate the use of the DLW method will increase exponentially in the near future. The GIRMSL has extensive experience in the operation of the isotope ratio mass spectrometers and the DLW method. For instance, the GIRMSL participated in the first whole-room calorimetric validation of the DLW method for energy expenditure measurement in man and is responsible for the development of many analytical techniques employed in the DLW method. The GIRMSL also was commissioned by the International Atomic Energy Agency in 1991 to produce several isotope reference materials including enriched 2H and 18O standards in order to provide internationally validated standards for the research community across the world. More importantly, the GIRMSL has been an OPEN laboratory since its inception in 1982 and has trained and supported numerous NIH-funded investigators at the Center and across the country. Recently, the GIRMSL has been selected by the NIH/NIA to be the Central DLW Laboratory to support their multi-center CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) phase 2 clinical trial. Currently, the GIRMSL is also supporting the NIH funded projects at the CNRC, Children's Hospital Boston, University of Califorina at Irvine, Washington University at St. Louis, Vanderbilt University, Tufts University, and Pennington Biomedical Research Center. Therefore, funding to purchase the Thermo Electron Delta V instrument will allow the GIRMSL to continue to provide training and analytical support to NIH funded investigators, both here at the CNRC and around the world. PUBLIC HEALTH RELEVANCE: To combat the epidemic of obesity in this country, it is important to know the amount of energy used or the amount of calorie consumed in the study population. Doubly labeled water is the reference method for estimation of energy expenditure or consumption under free-living conditions. The isotope ratio mass spectrometer requested is the most accurate and precise instrument to measure the stable hydrogen and oxygen isotopes in the doubly labeled water method and will be used for these measurements.